An analog radio frequency (RF) signal typically needs to be transformed once it has been received by an RF antenna. A common transformation is to change the amplitude of the RF signal. This transformation can be performed in an electronic circuit such as an RF signal chain. An RF signal chain transforms RF signals in preparation for processing by other electronic circuits. Any electronic circuit which performs this function is a RF signal chain. Various electronic elements including, but not limited to, an amplifier and an attenuator can be used in the RF signal chain to perform this transformation. An electronic element is said to have gain, which can be measured in decibels, if it causes a change in amplitude in the RF signal.
If the gain of the electronic element or electronic circuit can be changed during operation (while the RF signal is being transformed), the electronic element or circuit, respectively, is said to have variable gain. A variable gain RF signal chain is more useful than one which has a fixed gain and cannot be varied during operation. This is because RF signals are dynamic and, in certain applications, the RF signal amplitude can vary widely from signal to signal. In these applications, variable gain RF signal chains are useful for maximizing the signal-to-noise and distortion ratio of a RF signal, while preventing the overload of its electronic elements.
A variable gain RF signal chain typically comprises one or more variable gain electronic elements, such as variable amplifiers, variable attenuators, or more complex electronic elements incorporating a variable amplifier or variable attenuator such as a voltage gain amplifier (VGA). A VGA typically comprises two fixed amplifiers and a variable attenuator therebetween.
A known problem with changing the gain in a variable gain RF signal chain (through one of its electronic elements) during operation is that it can cause an unwanted phase shift in the RF signal being transformed. The phase shift is measured relative to the phase of the RF signal at the RF antenna or the input to the electronic circuit. A phase shift, especially an abrupt phase shift, is undesirable. It can cause an increase in bit errors in encoding methods dependent on phase information such as Quadrature Amplitude Modulation. The phase shift is caused by imperfect electronic components with non-zero capacitance. A change in gain causes a change in capacitance in the RF signal chain. A change in capacitance causes a phase shift in the RF signal.
It is known in the art that the phase shift can be reduced by minimizing the capacitance in the electronic components used in the variable gain electronic elements. Although it is not possible to eliminate entirely from the electronic circuit, capacitance can by reduced by minimizing the number of electronic elements contributing capacitance, minimizing the size of electronic elements contributing capacitance, and arranging the electronic elements contributing capacitance in a certain configuration. For example, capacitance in an attenuator (such as a simple resistive divider) can be reduced by limiting the size of the resistors and arranging the resistors in either a binary weighted configuration or cascade configuration.
A drawback to limiting the amount, size, and configuration of resistors, however, is that it also limits the attenuation range and frequencies of the simple resistive divider. Accordingly, an electronic circuit using this solution is limited in the amount of gain change.
It is desirable to provide an approach for varying the gain experienced by an RF signal that overcomes at least one drawback of known approaches.